The invention relates to a method for the operation of an optical sensor arrangement for detecting articles present in a monitored region using a pulsed light transmitter which transmits light pulses with a time spacing one after the other into the monitored region. A light receiver receives light and in the absence of articles to be detected receives substantially no light. When articles are in the monitored region the light receiver receives light and a processing stage connected to the light receiver transmits an article detection signal.
In the optical sensor arrangements with pulsed operation one is concerned in the broadest sense with light barriers, i.e. reflex light barriers, light sensors, distance sensors, visibility measuring devices etc. fall under this term. It is important that an optical sensor arrangement in accordance with the invention receives a light signal when an article to be detected is located in the monitored region and receives no light signal when no article to be detected is present in the monitored region. The invention can in particular be used with advantage when the non-detection of an article guided through the monitored region is unproblematic, i.e. does not for example involve a dangerous state, and in which the presence of an article in the monitored region may in no case be signalled when such an article is not present there.
The invention is primarily used where, for example, articles to be counted such as ampoules or packages pass one after the other through the monitored region and where the presence of an article in the monitored region may under no circumstances be simulated.
Optical sensor arrangements, in particular light barriers for the recognition of articles generally operate in accordance with the pulsed principle in which a light pulse is transmitted periodically for a short duration and the light reflected from the article is received by the light receiver and evaluated in the evaluation circuit. Each transmitted light pulse can for example have a time length of the order of magnitude of 1 to 10 .mu.s. A comparatively long pause, which can correspond to approximately ten to one hundred times the value of the transmitted pulse length, then follows each transmitted light pulse. The use of light pulses has two advantages. On the one hand, an improved signal/noise ratio is achieved in comparison with a continuous transmitter operation with the same power losses. On the other hand, the influence of constant light can be illuminated in simple manner by the insertion of a high pass filter into the received signal processing stage.
Problematic with such pulsed reflex light barriers is however the fact that disturbing signals which change with time, in particular disturbing pulses, can lead to a falsification of the evaluation result when they occur precisely during the sending of a transmitted light pulse and enter via an optical or electromagnetic path into the evaluation circuit. The disturbing signals can be both optical disturbing signals or also electromagnetic disturbances which can be coupled into the electronic part of the reflex light barrier following the optoelectronic conversion in the light receiver.
For this reason one has already attempted to restrict the influence of disturbing signals by filters which follow the light receiver which do not transmit every received signal as a detection signal. These measures have however the disadvantage that only a received light value averaged over a specific time period is evaluated. The effective switching frequency of the actual signal is thereby reduced in undesirable manner.